1. Field of Invention
The present invention relates to an actuator for an electrical parking brake system. More particularly, the present invention relates to an actuator for an electrical parking brake system, which can decrease the number of components and reduce noise generated in an operation of the actuator.
2. Description of Related Art
When a user parks a vehicle, the movement of the vehicle is generally restricted by manipulating a parking brake. If a cable is pulled by a parking lever in an ordinary parking brake, the force applied to the cable is distributed to a brake mechanism mounted to a wheel through an equalizer, and accordingly, the brake mechanism restricts the wheel, thereby preventing the movement of a vehicle.
However, the manipulation of the parking lever of the parking brake makes a user feel inconvenience. Accordingly, there has been developed an electrical parking brake (EPB) system which generates braking power for parking a vehicle, using power of a motor through button manipulation.
Recently, an EPB system for controlling the braking/releasing of a vehicle through simple button manipulation without lever manipulation has come into wide use. The EPB system eliminates user's inconvenience that user should manually manipulate a parking lever, thereby enhancing user's convenience.
The EPB system includes a cable puller type EPB system, a motor-on-caliper type (generally referred to as “caliper integrated” in the art) EPB system, a hydraulic EPB system, and the like. Among these EPB systems, the caliper integrated EPB system is widely used, which facilitates the mountability of the EPB system, using a method of integrating the EPB system with a caliper.
The EPB system generates braking power (target clamping force) by using together hydraulic braking through a brake pedal and motor power through an actuator as an electric power generation means. Thus, the EPB system can perform together hydraulic braking and electric power parking braking.
Particularly, in a case where the EPB system is integrated with a caliper, a gear device which converts a final output into a large load is applied while using a small-sized motor, so that the size of the caliper is not increased, thereby achieving a compact configuration.
The configuration of the EPB system will be described. The EPB system includes a button manipulated by a driver, a controller which outputs a control signal for generating braking power for parking a vehicle by receiving a button manipulation signal, an actuator which generates power according to the control signal output from the controller, and a caliper (EPB caliper) installed to surround a disc mounted to a wheel so that parking braking is implemented by restricting the disc and the wheel by means of power of the actuator.
FIGS. 1 and 2 are views showing the configuration of an electrical parking brake (EPB) system. In FIG. 1, reference numeral 10 denotes an actuator, and reference numeral 20 denotes a caliper. As shown in FIG. 2, the torque of the actuator is transmitted to a spindle 21, to be changed into a linear moving force which allows a nut member 22 to move forward. In a case where the linearly moving nut member 22 is contacted with an inner surface of a piston 23 by moving forward, the nut member 22 pushes the piston 23, and thus a disc 25 is compressed through a pad 24, thereby generating braking power.
However, the conventional EPB system has at least the following problems in its components including the actuator generating driving power (decelerated torque transmitted from a motor to the spindle), the spindle and the like.
Firstly, the degree of freedom is limited in setting a reduction ratio for enhancing rotary torque of the motor in terms of package layout and structure, and therefore, the load of the motor and the consumption of current are restrictive.
Secondly, when the power of a vehicle is off, the spindle takes charge of the function of self locking for preventing a brake from being pushed backward. Hence, the degree of freedom is limited in selecting specifications of the spindle, and a low-efficiency spindle is unavoidably used. Therefore, the current consumption of the actuator is excessive.
Thirdly, there is a problem of torque loss and excessive vibration, caused by the backlash of gear engagement. In addition, the number of gear components and unit cost are increased due to the use of a multi-gear configured with a spur gear, a helical gear and a planetary gear.
Fourthly, when the EPB system is operated, a frictional sound is generated considerably due to an excessive number of gear components. In addition, the number of components related to a gear box for fixing and assembling gears is excessive, and unit cost is increased.
Fifthly, since the number of components related to the gear and the gear box is excessive, it is highly likely that an assembling error and a failure may occur.
In order to solve such problems, Korean Patent Application No. 10-2013-0082021 (2013.7.18) discloses an actuator provided with a simpler configuration using a worm-gear-type gear having a high reduction ratio, rather than an actuator provided with a complicated configuration using a gear combination of a spur gear, a helical gear and a planetary gear.
In the actuator of the prior patent, a driving gear on a motor shaft, which is rotated in the driving of a motor, is formed in the shape of a worm gear, and a first gear portion of a transmission member, which is engaged with the driving gear, is formed in the shape of a spur gear (worm wheel).
In addition, a second gear portion of the transmission member, which transmits torque to an output gear, is formed in the shape of a worm gear, and the output gear engaged with the second gear portion of the transmission member is formed in the shape of a spur gear (worm wheel).
As described above, the driving gear on the motor shaft and the second gear portion of the shaft-shaped transmission member have the configuration of a double worm gear formed in the shape of a worm gear, and the primary and secondary reductions of speed are made through the driving gear and the second gear portion.
That is, the reduction of speed is made in a process of transmitting power between the driving gear and the first gear portion (the primary reduction of speed) and a process of transmitting power between the second gear portion and the output gear (the secondary reduction of speed).
However, in the actuator of the prior patent, the torque transmission efficiency is not good due to the excess of distortion torque of the gear shaft (transmission member), the deformation of a shaft system, the occurrence of backlash between gears, and the like in the application of a load.
Further, since stress generated in a specific part is excessive in the application of a load, the abrasion resistance of the actuator is lowered, and the lifespan of the actuator is reduced. In addition, the frictional sound (operation sound) of the gears is large due to the excess of stress and the instability of the shaft system.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.